1. Field of the Invention
The present invention relates to a piston head of a hydraulic injection molding machine, and more particularly to a piston head of a hydraulic injection molding machine capable of reducing a maximum stress applied to a tie rod screwed with the piston head.
2. Description of the Related Art
Generally, an injection molding machine includes an injection device and a clamping device. The injection device melts a resin and injects the melted resin into a mold under high pressure. The clamping device opens/closes the mold and applies a clamping force for combining the mold not to be opened by the high pressure melted resin injected to the mold. This injection molding machine is disclosed in Korean Laid-open Patent Publication No. 10-2005-0097366.
The clamping force is generated from a hydraulic cylinder. As shown in FIG. 1, a hydraulic pressure is applied to a piston head 10 screwed with a tie rod 20 inside a hydraulic cylinder (not shown) to generate a clamping force. That is, a hydraulic pressure is acted to the piston head 10 and a fixed template (not shown) at an opposite side of the piston head 10 at the same time to generate a clamping force. The hydraulic pressure applied to the piston head 10 is transferred to the tie rod 20.
While the hydraulic pressure is transferred as mentioned above, a stress is concentrated on a first screw 22 of the tie rod 20. That is to say, a load caused by the hydraulic pressure is not uniformly applied to the first screw 22 but a maximum stress is applied to a first root 23 of the first screw 22. It is because the hydraulic pressure is directly acted on the first screw 22 as a compressing stress while being transferred alone an arrow 30 from the piston head 10. Reference numeral 12 designates a second screw formed on the piston head 10.
A stress distribution on the first screw 22 is shown in FIG. 6. In FIG. 6, the term ‘root number’ indicates the sequence of roots from a right side of the first screw 22 formed on the tie rod 20.
Meanwhile, since the load applied to the first screw 22 is repeated, a fatigue limit of material is also considered for preventing a fatigue failure together with a tensile strength and a safety factor when determining dimensions of the tie rod 20.
Since dimensions of the tie rod 20 should be determined based on a maximum stress, as the maximum stress is increased, the tie rod 20 should have a greater diameter and a preform used for making the tie rod 20 should have a greater size.
In order to solve the above problem, as shown in FIG. 2, a piston head 10a having a stress relieving groove 14a has been proposed. The stress relieving groove 14a is formed along a circumference of the tie rod 20 in front of the piston head 10a. Such a stress relieving groove 14a is disclosed in U.S. Pat. No. 4,571,135 and so on.
However, if the piston head 10a is used in a place where a hydraulic pressure is applied, the hydraulic pressure is also applied to the stress relieving groove 14a. by which effects such as maximum stress reduction or stress regularization cannot be expected. According to numerical analysis results, a stress on a first root 23 of the tie rod 20, namely a maximum stress, is increased about 5%, and a minimum stress among stresses applied to second and later roots is decreased, thereby increasing irregularity of stress.
If a maximum stress is decreased by means of regularization of stresses applied to the first screw 22, it would be possible to enhance a safety factor, lower possibility of fatigue failures, decrease a diameter of the tie rod 20 and reduce material costs and production costs for making the tie rod 20.